2,6-Xylenol is the starting material for the production of polyphenyleneoxide, a valuable polymer used in the art for several applications.
The limited availability of 2,6-xylenol deriving from sources such as the heavy fractions from the distillation of coal, and the problems related to the separation from such sources of 2,6-xylenol at a suitable purity, have promoted the development of processes for the chemical synthesis of 2,6-xylenol. Presently, 2,6-xylenol is produced nearly exclusively by the catalytic methylation of phenol with methanol. In this reaction, together with the desired product 2,6-xylenol, also not negligible quantities are formed of cresols and xylenol isomers of 2,6-xylenol. In the reaction mixture also unconverted reactants, water and several byproducts are present. By separating such a reaction mixture by distillation and rectifying, it is possible to obtain a 2,6-xylenol with a purity of about 95-98%.
However, in the production of polyphenylene oxide by the oxidative polymerization of 2,6-xylenol, a very pure 2,6-xylenol is required, in any case with a purity higher then 99%. Thus, 2,6-xylenol obtained by chemical synthesis undergoes, according to the art known, purification processes, by means of crystallization, mainly from solvents. Selective solvents used for this purpose are, e.g., aliphatic organic acids and water/ethylene glycol mixtures.
However, the purification of raw 2,6-xylenol on the basis of the crystallization from solvent is generally a complex and also expensive process, due to the need of recovering and regenerating the solvent used for this purpose.
In the disclosure of the French Pat. No. 1.583.244, a process is disclosed for the purification of raw 2,6-xylenol, which consists essentially in heating said solid raw 2,6-xylenol, slowly and gradually, so as to cause only a partial melting of it. In this way, a molten liquid fraction of raw 2,6-xylenol is separated, which is richer in impurity than the fraction which did not undergo the melting.
Such a way of operation, if it on one hand eliminates the drawbacks arising from the use of the solvents, on the other hand does not allow a purified 2,6-xylenol to be obtained at a purity level as high as desirable.